Sensors are utilized in a variety of sensing applications, such as, for example, detecting and/or quantifying the composition of matter, detecting and/or quantifying the presence of a particular substance from among many substances, and detecting and/or quantifying a mass flow rate of a substance. The industrial, commercial, medical, and automotive industries in particular require many ways to quantify the amount of gaseous and liquid mass flow rates. For example, in the medical industry, an airflow sensor is often employed to monitor and/or control a patient's breathing. Two examples of this include sleep apnea devices and oxygen conserving devices. Similarly, airflow sensors are often employed in microcomputer cooling units to detect the presence and amount of local airflow in, through, and around the cooling units.
Historically, mass flow sensors have been constructed with one temperature-sensing resistor “upstream” and one temperature sensing resistor “downstream,” where “upstream” and “downstream” generally indicate the direction of mass flow. One advancement in mass flow sensors in microchip environments, the “Wheatstone bridge” circuit, is often configured with external, off the chip, resistors. This historical configuration can be improved by implementing a full Wheatstone bridge, all four resistors, each a temperature sensing resistor, on the sensing chip, to allow for a larger signal to noise ratio and better immunity to ambient temperature noise.
Wheatstone bridges can be used to detect mass flow. For example, in a “full” Wheatstone bridge configuration, all four legs comprise variable resistors. In one configuration, resistive temperature detectors—resistors that vary in resistance with temperature—are used in each leg. A heating element situated between the two sides creates a roughly even thermal distribution about the heating element. As air, for example, passes from one side to the other side of the bridge, heat is conducted away from the “upstream” side to the “downstream” side, cooling the upstream side and heating the downstream side.
As the resistance of the two sides varies with temperature, the resultant temperature differential between the two sides causes a measurable voltage difference between the two sides. This voltage difference can be correlated to the difference in temperature. As the temperature change is a function of the air mass flow rate, the voltage difference can also be correlated to the mass flow rate.
However, previous full Wheatstone bridge configurations also often incur a low signal to noise ratio, particularly for very high or very low flow rates. A low signal to noise ratio reduces the accuracy and resolution of the bridge measurements and can cause difficulties in quantifying the mass flow rates under investigation.
Therefore, what is required is a system, apparatus, and/or method that provides an improved sensitivity to high and/or low flow rates that overcomes at least some of the limitations of previous systems and/or methods.